for-each-ref: introduce %(HEAD) asterisk marker

for-each-ref: introduce %(HEAD) asterisk marker

'git branch' shows which branch you are currently on with an '*', but
'git for-each-ref' misses this feature.  So, extend its format with
%(HEAD) for the same effect.

Now you can use the following format in for-each-ref:

  %(HEAD) %(refname:short)

to display an asterisk next to the current ref.

Signed-off-by: Ramkumar Ramachandra <redacted>
Signed-off-by: Junio C Hamano <redacted>